Physics and Engineering of Radiation Detection

8.7. Analog to Digital Conversion 499

full dynamic range of the system. The lowest number that the ADC can assign to a
meaningful analog input determines its resolution.
The resolution of an ADC is generally represented inbits.An-bit ADC ideally
has a resolution of
V
V

=

1

2 n

. (8.7.1)

Example:

Determine the resolutions of an 8-bit and a 16-bit ADC.

Solution:

The 8-bit ADC as a resolution of

V

V

=

1

2 n

=

1

28

=3. 90 × 10 −^3.

Similarly the resolution of the 16-bit ADC is

V

V

=

1

216

=1. 52 × 10 −^5.

A.4 Nonlinearity...........................

Any nonlinearity in the analog to digital conversion process can be a serious problem
at least for high resolution detection systems. It is therefore extremely important
that the linearity of the ADC used in the system is ensured. Unfortunately the
linearity of the analog to digital conversion process does not depend just on the ADC
design and circuitry but also on the shape of the analog pulse and its duration.
There are two types of nonlinearities that must be eliminated or at least mini-
mized in a practical ADC. These are generally referred to asdifferentialandintegral
nonlinearity.

Differential Nonlinearity:Differential nonlinearity is a measure of the uni-

formity of the increments through which an ADC goes during the conversion

process.

Integral Nonlinearity:If the ADC counts are not linearly proportional to

the analog pulse amplitude then it is said to have integral nonlinearity.

A.5 Stability

It is generally desired that the physical parameters, on which the analog to digital
conversions depend, do not significantly change with time. Therefore stability is one
of the important criteria for choosing an ADC.